The invention relates to a valve drive of an internal combustion engine for actuating a gas-exchange valve. Its motion follows the lift of a cam and also the lift of a hydraulic force-applying device superimposed on and independent of the lift of the cam. For this purpose, a piston of the force-applying device can move relative to a housing of the force-applying device from a first end position to a second end position in a pressure chamber formed by the piston and the housing through timed-variable feeding of a pressure-adjustable hydraulic medium from a hydraulic medium line.
Valve drives according to this class, in which the lift of the gas-exchange valve is comprised of superimposing a lift originating from the cam and a variable, adjustable lift of a hydraulic force-applying device, which acts on the motion of the gas-exchange valve independent of the cam, are known in the state of the art. For example, DE 101 56 309 A1 describes a cup-tappet valve drive with a hydraulic force-applying device. This is used to superimpose a lift generated by the cam on a lift of the gas-exchange valve independent of the cam. For this purpose, between the inside of the cup base and the valve shaft there is a pressure piston, whose relative motion relative to the cup tappet is generated through a volume change of a pressure chamber bordering the pressure piston. The pressure chamber is connected, on one side, via channels in the interior of the cup tappet and also in the tappet guide of the internal combustion engine to a hydraulic medium supply that is adjustable in pressure or volume flow.
In DE 43 18 293 A1, also according to this class, a finger lever drive with a pivot support is proposed, whose bearing point for the finger lever can be lowered by regulating the hydraulic medium out of the pressure chamber of the force-applying device by means of a control valve. By lowering the bearing point, the cam lift is sub-divided kinematically onto the bearing point and the gas-exchange valve, which reduces the lift transmitted to the gas-exchange valve.
Although with the previously mentioned valve drive an essentially variable influence of the valve lift originating from the cam is already possible, wherein partially also means for braking the piston motion are provided for reaching the end positions, the previously known systems have a few disadvantages. For example, the piston of DE 101 56 309 A1 is embodied as a stepped piston, which forces hydraulic medium from an annular space located on the cup bottom with a cylindrical annular section. For reaching the end position, the piston is here braked by forcing the hydraulic medium out of the annular space via guide gaps between the annular section and annular space. Such a construction, however, requires the double fitting of the components, resulting in the hydraulic force-applying device being associated with considerable production and quality-assurance expense and consequently high manufacturing costs. Moreover, the piston is then prevented from leaving the end position at a high acceleration and thus as quickly as possible, because the annular space first must be refilled with hydraulic medium via the narrow guide gaps.
In DE 43 18 293 A1, a ball check valve is located between the housing of the pivot support and the hydraulic medium supply. This is arranged, however, in the cylinder head of the internal combustion engine in a way that is not easy to assemble and is also limited in throughput according to principle. In this respect, here a high acceleration of the piston can be realized only to a limited extent when it leaves its end position.
In the two publications noted above, the braking profile of the piston when reaching the end position dependent on the viscosity and thus, in particular, on the temperature of the hydraulic medium, is further to be viewed as disadvantageous. Both forcing the hydraulic medium via annular gaps, as provided in DE 101 56 309 A1, and also connecting the pressure chamber to a relatively long choke line according to DE 43 18 293 A1 leads to a considerable dependency of the braking profile on the viscosity of the hydraulic medium. This dependency, however, is in no way desired. In addition, the very wide operating temperature span of the internal combustion engine would lead to extremely different braking profiles of the piston, which could be equalized only with high electro-hydraulic control expense.